1. Field of the Invention
The present invention relates generally to a wireless mobile communication system, and in particular, to a system and method for performing a handover for a Mobile Station (MS).
2. Description of the Related Art
Provisioning of services with a variety of Quality of Service (QoS) levels at about 100 Mbps to users will be necessary in a future-generation communication system called a 4th Generation (4G) mobile communication system. The 4G communication system is envisioned as a new communication system that supports user mobility and QoS to Wireless Local Area Network (WLAN) providing relatively high data rates and a Wireless Metropolitan Area Network (WMAN). A major 4G communication system is known as the Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system.
With reference to FIG. 1, the configuration of a multi-cell IEEE 802.16e communication system will be described as one of the IEEE 802.16 communication systems.
FIG. 1 illustrates the configuration of the typical IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system has a plurality of cells 100 and 150, Base Stations (BSs) 110 and 140 having the cells 100 and 150 under their control, respectively, and a plurality of MSs 111, 113, 130, 151, and 153.
The MS 130 is located at the boundary between the cells 100 and 150, i.e., in a handover region. Thus, support of handover for the MS 130 is equivalent to support of mobility for the MS 130.
Since the IEEE 802.16e communication system supports mobility to an MS, it can move from a current BS (i.e. a serving BS) to a neighbor BS.
The process from selection of the serving BS by the MS until before handover will be described below.
Upon power-on, the MS monitors a total frequency band and detects a pilot channel signal having the highest pilot Carrier-to-Interference and Noise Ratio (CINR). The MS determines that the BS which sent the pilot channel signal is its serving BS. Then the MS receives pilot signals from the serving BS and neighbor BSs and measures the CINRs of the pilot signals. If the CINR of a neighbor BS is higher than that of the serving BS, the MS operates for a handover to the neighbor BS. This step of measuring the channel status of the serving BS and the neighbor BSs is called scanning. With reference to FIG. 2, an MS-initiated scanning in the typical IEEE 802.16e communication system will be described.
FIG. 2 is a diagram illustrating a signal flow for the MS-initiated scanning in the typical IEEE 802.16e communication system.
A scanning request can be initiated by a BS or an MS. In the former case, the BS may request the MS to scan neighbor BSs in order to distribute its load, while in the latter case, the MS may request scanning to the BS when the CINR of a current channel is lower than a predetermined threshold.
Referring to FIG. 2, a first BS 220 (BS #1) is a serving BS which periodically sends a Mobile_Neighbor-Advertisement (MOB_NBR-ADV) message to an MS 200 in step 202. BS #1 broadcasts information about a second BS 240 (BS #2) and a third BS 260 (BS #3) by the MOB_NBR-ADV message. The MOB_NBR-ADV message is formatted as defined in the IEEE 802.16e/Document 8, the contents of which are incorporated herein by reference.
When the MS 200 desires to scan the neighbor BSs after receiving the MOB_NBR-ADV message, it sends a Mobile_Scanning Interval Allocation-Request (MOB_SCN-REQ) message to BS #1 in step 204. The MOB_SCN-REQ has a configuration as defined in the IEEE 802.16e/Document 8.
Upon receipt of the MOB_SCN-REQ message, BS #1 replies with a Mobile_Scanning Interval Allocation-Response (MOB_SCAN-RSP) message containing scanning information (such as scan iteration information, etc.) to the MS 200 in step 206. The configuration of the MOB_SCN-RSP is also defined in the IEEE 802.16e/Document 8.
The MS 200 scans the pilot CINRs of the neighbor BSs for N frames M frames after receiving the MOB_SCN-RSP message in steps 208 and 210. The parameters M and N are assumed, for convenience sake. How many times the MS 200 is to scan is determined by scan iteration information included in the scanning information.
The MS 200 reports pilot CINR measurements acquired during the scanning to BS #1 by a Mobile_Scanning-Report (MOB_SCN-REP) message in step 211. In step 212, the MS receives data traffic from BS #1 during a frame period indicated by interleaving interval information.
In step 214 through step 218, the MS 200 repeats the pilot CINR measurement and the data traffic reception a predetermined number of times, i.e., as many times as indicated by the scan iteration information.
As described above, to support handover in the IEEE 802.16e system, the MS 200 measures the pilot CINRS of the serving BS 220 and the neighbor BSs. If the pilot CINR of the serving BS 220 is lower than that of a neighbor BS 240 or 260, the MS 200 request a handover (to one of the target BSs 240 or 260) to the serving BS 220.
Owing to the development of communication technology, MSs demand large-capacity and diverse services. In this context, BSs must satisfy the demand by operating in both a Narrow frequency Band (NB) and a Wide frequency Band (WB). Also, the BSs may use various frequency modes, for example, a combination of Frequency Division Duplex (FDD) and Time Division Duplex (TDD). When a BS uses at least two different frequency bands and/or at least two different frequency modes (i.e., operates in a dual mode), an MS communicating with the corresponding BS must operate the frequency bands and/frequency modes in a similar fashion.
Accordingly, MSs operating in a dual mode or other similar environment (e.g., the NB and/or WB environments) scan the NB and the WB individually, and/or scan in the individual frequency modes. This results in the increase of scanning time and hardware/software complexity. In addition, the MS cannot receive data traffic during the scanning period. Accordingly, there is a need for a technique for enabling fast, successful handover through efficient scanning of an MS in a communication system using different frequency bands and/or different frequency modes.